Minerals as buffers

In a first example of how minerals can buffer a fluid s chemistry, we consider how a hypothetical groundwater that is initially in equilibrium with calcite (CaCC F) at 25 °C might respond to the addition of an acid. In REACT, we enter the commands 

In a second calculation, we trace the same path while maintaining the fluid in equilibrium with the calcite. To do so, we enter the command 

The overall reaction for the earliest portion of the reaction path, where HCOJ is the predominant carbonate species, is 

By this reaction, the fluid becomes more acidic with the addition of HC1. With decreasing pH, the C02(aq) species quickly comes to dominate HCO3. At this point, the principal reaction becomes, 

According to this reaction, adding HC1 to the fluid no longer affects pH. Instead, calcite dissolves to neutralize the acid, leaving Ca++ and C02(aq) in solution (Fig. 15.7). 

4 Use of reaction modeling to derive a fluid s carbonate concentration from its titration alkalinity, as applied to an analysis of Mono Lake water. When the correct HCOj total concentration (in this case, 25,100 mg/kg) is set, the final pH matches the titration endpoint. 

The only pH buffer in the calculation is the small concentration of carbonate species in solution. The buffer is quickly overwhelmed and the fluid shifts rapidly to acidic pH, as shown in Fig. 13.6. The dominant reaction, 

5 Concentrations of species in buffer reactions that contribute to the titration alkalinity of Mono Lake water, plotted against pH. 

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It is assumed that in such rocks oxides—hematite and magnetite—were essential minerals as well as siderite. The association of siderite with magnetite and fayalite in equilibrium with graphite in reducing conditions has already been examined now it remains to analyze in detail the particulars of metamorphism in oxidizing conditions in equilibrium with a hematite-magnetite buffer. 

As a second example of mineral-controlled buffer capacity, consider the reaction in pure water between the clays kaolinite and illite (here assumed the same as muscovite), which may be written... 

According to Findley, Swern, and Scanlan,22 work with peroxyacetic acid solutions is best carried out at reaction temperatures of 20-25° with reaction times as short as possible strong mineral acids (H2S04) must not be present in the reaction mixture, for which reason it is recommended that sodium acetate be added as buffer. 

Most autotrophic bacteria seem to grow best in soils that are well supplied with humus, which serves as a source of carbon dioxide, mineral nutrients, and acts as buffer against acids and alkalies. The most studied autotrophic genera are Nitrosomonas which oxidizes ammonia to nitrite, and Nitrobacter that converts the nitrite to nitrate. 

Generally speaking, gluconic acid and its salts (metal gluconates) are used in the formulation of food, pharmacentical, and hygienic products. They are also used as mineral supplements to prevent the dehciency of calcium, iron, and so forth and as buffer salts (Ramachandran et al. 2006). 

Reactions were initiated by adding cell suspension (final cell concentration 2 X 10 cells mL" ) and Fe (hydr)oxide (surface area of 100 m L ) to each reactor and bringing the volume to approximately 100 mL with Cr-free medium. The initial flow rate of media into the cell before Cr addition varied between 10 and 11 mL h Following Cr addition, the pump speed was decreased until a steady-state Fe(II) concentration was maintained in the effluent. The flow-rate varied from about 1.5 to 4.5 mLh at steady-state conditions depending on both Fe mineral and buffer conditions, yielding retention times that ranged from 22 to 67 h. Samples collected were analyzed for soluble Fe and Cr as described below. 

Aluminosilicates abound in most rocks considered suitable for the disposal of radioactive wastes (acidic-basic igneous and metamorphic rock types, and clays), so that the abundance of these minerals as potential buffers of pH should not be a limiting factor. Calcite may be less abundant in fractured hard rock systems than sedimentary rocks such as clays. 

For example, if the amount of ferrous iron minerals present in repository backfill and fracture minerals (represented by FeC03(s) in Fig. 1(a)) is much greater than the amount of O2 remaining after closure, then with time, all O2 will be reduced to H2O by these minerals, producing iron hydroxide in the process. This would ensure that the reducing intensity would return to values at least as low as the redox potential of the Fe(0H)3(s)/FeC03(s) couple (near —0.05 V). This is below the threshold for corrosion of either copper or uranium oxide by O2. It is also shghtly above the threshold for sulphide production by sulphate reduction (—0.2 V). The presence of ferrous minerals thus buffers the redox intensity of the repository to conditions that are favourable for repository performance. 

Boles and Franks 1979). However, it is our position that although these relatively slow aluminosilicate reactions contribute to the alkalinity, they do not buffer the alkalinity until all relatively fast-reacting minerals such as carbonates are eliminated from the fluid/rock system. It is the weak acids and bases (dominately carboxylic acid anions, HCO3, and HS ) in formation waters, coupled with rapidly reacting minerals, such as the carbonate minerals, that buffer formation water pH. 

In hemodialysis, blood (feed stream) contacts the membrane allowing solutes such as metabobc substances (e.g., urea, uric acid, creatinine) and salts/minerals (e.g., NaCl, KCl, calcium, phosphorous, magnesium, sulfate) to be removed. The permeate stream (refeued to as the dialysate ) usually contains levels of salts/minerals (as noted above) similar to that in blood so that an imbalance in the natural concentration in blood is not abraptly modified. Bicarbonate is also added to the permeate stream to act as a buffer. A modest pressure gradient is applied to the membrane... 

Alkaline solutions of mononitroparaffins undergo many different reactions when stored for long periods, acidified, or heated. Acidification of solutions of mononitro salts is best effected slowly at 0°C or lower with weak acids or buffered acidic mixtures, such as acetic acid—urea, carbon dioxide, or hydroxyl ammonium chloride. If mineral acids are used under mild conditions, eg, dilute HCl at 0°C, decomposition yields a carbonyl compound and nitrous oxide (Nef reaction). 

When a forest system is subjected to acid deposition, the foliar canopy can initially provide some neutralizing capacity. If the quantity of acid components is too high, this limited neutralizing capacity is overcome. As the acid components reach the forest floor, the soil composition determines their impact. The soil composition may have sufficient buffering capacity to neutralize the acid components. However, alteration of soil pH can result in mobilization or leaching of important minerals in the soil. In some instances, trace metals such as Ca or Mg may be removed from the soil, altering the A1 tolerance for trees. 

Addition of acetic or mineral acid to skimmed milk to reduce the pH value to 4.6, the isoelectric point, will cause the casein to precipitate. As calcium salts have a buffer action on the pH, somewhat more than the theoretical amount of acid must be used. Lactic acid produced in the process of milk souring by fermentation of the lactoses present by the bacterium Streptococcus lactis will lead to a similar precipitation. 

Hydrazino compounds can react with one or two equivalents of arenediazonium ions. In reactions of arylhydrazines without substituents at the P-nitrogen in mineral acid media the initial product, the 1,4-diaryltetraz-1-ene (6.24), disproportionates rapidly into aryl azide and amine (Scheme 6-17). As shown by 15N labeling experiments (Clusius and Craubner, 1955), equal amounts of all the products shown in Scheme 6-17 are obtained. In acetate buffer the reaction is regiospecifically different. The diazonium ion attacks the a-, not the P-nitrogen, and a 1,3-diaryltetraz-l-ene is formed (6.25). 

The H2S concentration of hydrothermal solution is plotted in Fig. 2.33. Based on these data, we can estimate the temperature of hydrothermal solution buffered by alteration mineral assemblages such as anhydrite-pyrite-calcite-magnetite and pyrite-pyrrhotite-magnetite for Okinawa fluids. 

The reactivity of steam can be reduced via pH control. The injection or addition of a buffer such as ammonium chloride inhibits the dissolution of certain mineral groups, controls the migration of fines, inhibits the swelling of clays, controls chemical reactions in which new clay minerals are formed, and... 

During the lifetime of a root, considerable depletion of the available mineral nutrients (MN) in the rhizosphere is to be expected. This, in turn, will affect the equilibrium between available and unavailable forms of MN. For example, dissolution of insoluble calcium or iron phosphates may occur, clay-fixed ammonium or potassium may be released, and nonlabile forms of P associated with clay and sesquioxide surfaces may enter soil solution (10). Any or all of these conversions to available forms will act to buffer the soil solution concentrations and reduce the intensity of the depletion curves around the root. However, because they occur relatively slowly (e.g., over hours, days, or weeks), they cannot be accounted for in the buffer capacity term and have to be included as separate source (dCldl) terms in Eq. (8). Such source terms are likely to be highly soil specific and difficult to measure (11). Many rhizosphere modelers have chosen to ignore them altogether, either by dealing with soils in which they are of limited importance or by growing plants for relatively short periods of time, where their contribution is small. Where such terms have been included, it is common to find first-order kinetic equations being used to describe the rate of interconversion (12). 

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